An ink jet printer produces images on a receiver medium by ejecting ink droplets onto the receiver medium in an imagewise fashion. The advantages of non-impact, low-noise, low-energy use, low cost and the capability to print on plain paper are largely responsible for the wide acceptance of ink jet printers in the marketplace.
The quality of images printed by ink jet printers is related to the absorption of inks into an ink receiver which can be plain paper, coated paper, transparent film and the like. Ink absorption capability of the ink receiver is characterized by properties of the receiver, such as the amount and the rate of the ink absorption. These properties are determined by the type of receiver. For example, it may be desirable to select specialty receivers coated with ink absorption layers which can absorb more ink at a faster rate (i.e., shorter "drying" time) than plain paper.
However, a problem associated with ink jet printing is excessive laydown of inks on the ink receiver. That is, when inks are placed on the receiver at an amount or rate higher than the receiver can accept, image defects can occur. For example, image artifacts can occur when neighboring ink pixels come in contact with each other and coalesce. This type of image artifact is commonly referred as "ink beading". Coalescence of ink pixels on the receiver causes inks to diffuse or flow among ink pixels and results in a non-uniform or mottled appearance of the printed image. This ink diffusion problem is most visible at the boundaries of printed areas comprising different colors, where the ink of one color diffuses into the adjacent area of a different color ink to form a finger-shaped pattern. This latter image defect is commonly referred to as "color bleeding".
Prior art techniques attempt to overcome ink beading and color bleeding by reducing the number of ink pixels printed in each printing pass. In this regard, U.S. Pat. No. 4,748,453 discloses a technique involving printing an image area with at least two passes. In each pass, the ink pixels are printed in a checkerboard pattern of diagonally adjacent pixels. The final image is formed by the sum of the complimentary checkerboard patterns in different printing passes. A disadvantage of this technique is the increased printing time caused by multiple printing passes.
U.S. Pat. Nos. 5,012,257 and 5,602,572 disclose techniques which define a spatial cell (or superpixel) of multiple pixels. A subset of the pixels within the cell are not printed (or "turned off") to reduce the coalescence of neighboring ink spots. These techniques have the disadvantage of reduced spatial resolution in the printed images.
U.S. Pat. No. 5,633,662 disclosed computation techniques that converts the pixel values in the source image file, that requires higher amount of ink laydown on the ink receiver, to a new image file that corresponds to reduced ink laydown. This method address the above described problem for ink jet printing that prints a two density levels (single-bit) for each color.
Therefore, there has been a long-felt need to provide an ink jet printing apparatus and method accommodating printing mode control for printing multiple-density levels on a receiver medium in a manner solving the problems of ink beading and color bleeding while avoiding excessive printing time and excessive ink laydown.